"He tried to go into the bar but did not make it through the door, someone took the gun from him and he got shot and is dead", a post from Facebook user James Williams said.
There are also reports the shooter was a man who was armed with a rifle. The shooter was later killed by the police.
Connor Betts, a 24-year old white man, was identified as the shooter according to Dayton Police Lt. Col. Matt Carper.
A spokesperson for Miami Valley Hospital said 16 victims were being treated there, NBC News reported.
Dayton Mayor Nan Whaley said the shooter was wearing body armor and had additional high-capacity magazines.
The FBI is assisting with the investigation, Carper said.
Sunday's shooting in Dayton is the 22nd mass killing of 2019 in the us, according to the AP/USA Today/Northeastern University mass murder database that tracks homicides where four or more people were killed - not including the offender.
"Dayton has been through a lot already this year, and I continue to be amazed by the grit and resiliency of our community", Whaley said. "When you came outside as soon as you get to front street you see the bodies and you knew that this was different, and you knew it was something that you never thought of experiencing never experienced, wouldn't want anyone to experience", the 23-year-old said. Kettering Health Network spokeswoman Elizabeth Long said multiple victims from a shooting had been brought to system hospitals, but she didn't have details on how many.
At least one person was shot in the abdomen and at least two people were taken into surgery. The others were adults ranging in age from 25 to 57.
Most of the injuries were gunshot wounds, but there were others, like cuts received while people were fleeing the scene. Twenty seven others were injured, with four people in serious condition and one person in critical, medical authorities said.
Gov. Mike DeWine said in a statement he was "absolutely heartbroken" and ordering flags flown at half-staff.
DeWine said he had been briefed on the Dayton shooting by the Ohio Department of Public Safety. Thank you to our first responders for all that you've done. "We will share updates as we have more information".
Authorities gave no details about the circumstances of the shooting except that it occurred in the city's Oregon District, an historic neighborhood known for its nightclubs, bars, art galleries and shops.
Carper said OR was "a very safe part of downtown" Dayton and very popular with visitors.